The transformation of cyclodextrins (CDs) and alkanes from separated monomers to inclusion complexes at the interface is illustrated by analyzing the evolution of interfacial tension along with the variation of interfacial area for an oscillating drop.

The dynamic interfacial tensions (IFTs) of two novel zwitterionic surfactants with different hydrophobic groups, alkyl sulfobetaine (ASB), and xylyl substituted alkyl sulfobetaine (XSB), against kerosene, crude oil, and model oils containing crude oil fractions, such as resins, asphaltenes, saturates, aromatics, and acidic fractions, have been investigated by a spinning drop interfacial tensiometer. The experimental results show that XSB solutions show higher interfacial activity than ASB against kerosene because of the larger size of the hydrophobic part of the XSB molecule. The petroleum acids have high interfacial activity and can adsorb onto the interface. For ASB solutions, the synergism mixed adsorption of betaine and acid molecules lowers IFT values. On the one hand, the partly displacement of XSB molecules by petroleum acid at the interface results in the increase of IFTs. Therefore, resins, aromatics, and acidic fractions show strong effects on IFTs of betaine solutions. On the other hand, asphaltenes and saturates have little effect on interfacial properties. Moreover, the hydrophilic part of the betaine molecule at the interface may vary its orientation from vertical to flat with aging time. Therefore, the dynamic IFT curves of ASB solutions against model oils show “V” shape for resins, aromatics, and acidic fractions. GRAPHICAL ABSTRACT

The adsorption behaviors of extended anionic surfactants linear sodium dodecyl(polyoxyisopropene)₄ sulfate (L-C₁₂PO₄S), branched sodium dodecyl(polyoxyisopropene)₄ sulfate (G-C₁₂PO₄S), and branched sodium hexadecyl(polyoxyisopropene)₄ sulfate (G-C₁₆PO₄S) on polymethylmethacrylate (PMMA) surface have been studied. The effect of branched alkyl chain on the wettability of the PMMA surface has been explored. To obtain the adsorption parameters such as the adhesional tension and PMMA-solution interfacial tension, the surface tension and contact angles were measured. The experimental results demonstrate that the special properties of polyoxypropene (PO) groups improve the polar interactions and allow the extended surfactant molecules to gradually adsorb on the PMMA surface by polar heads. Therefore, the hydrophobic chains will point to water and the solid surface is modified to be hydrophobic. Besides, the adsorption amounts of the three extended anionic surfactants at the PMMA–liquid interface are all about 1/3 of those at the air–liquid interface before the critical micelle concentration (CMC). However, these extended surfactants will transform their original adsorption behavior after CMC. The surfactant molecules will interact with the PMMA surface with the hydrophilic heads towards water and are prone to form aggregations at the PMMA–liquid interface. Therefore, the PMMA surface will be more hydrophilic after CMC. In the three surfactants, the branched G-C₁₆PO₄S with two long alkyl chains exhibits the strongest hydrophobic modification capacity. The linear L-C₁₂PO₄S is more likely to densely adsorb at the PMMA–liquid interface than the branched surfactants, thus L-C₁₂PO₄S possesses the strongest hydrophilic modification ability and shows smaller contact angles on PMMA surface at high concentrations.

In order to explore the mechanism responsible for the interactions in the surfactant–polymer composite flooding and broaden the application range of the binary system in heterogeneous oil reservoirs, in this paper, the influences of different surfactants on the viscosity of two polymers with similar molecular weights, partially hydrolyzed polyacrylamide (HPAM) and hydrophobically modified polyacrylamide (HMPAM), were studied at different reservoir environments. In addition, the relationship between the surfactant–polymer synergistic effects and oil displacement efficiency was also investigated. The experimental results show that for HPAM, surfactants mainly act as an electrolyte to reduce its viscosity. For HMPAM, SDBS and TX-100 will form aggregates with the hydrophobic blocks of polymer molecules, reducing the bulk viscosity. However, zwitterionic surfactant aralkyl substituted alkyl sulfobetaine BSB molecules can build “bridges” between different polymer molecules through hydrogen bonding and electrostatic interaction. After forming aggregates with HMPAM molecules, the viscosity will increase. The presence of two polymers all weakened the surfactant oil–water interfacial membrane strength to a certain extent, but had little effect on the interfacial tension. The synergistic effect of the “bridge” between HMPAM and BSB under macroscopic conditions also occurs in the microscopic pores of the core, which has a beneficial effect on improving oil recovery.